1. Field of the Invention
The present invention generally relates to a device for spinal fixation, and in particular to an adjustment device for many types of spinal implants. The device finds particularly suitable applications in spinal fusion devices such as a connector for coupling elongate members (such as spinal rods), plates, and the like, as well as in adjustable vertebral spacers for intervertebral fusion devices, corpectomy devices, and other vertebral prostheses.
2. Background
The spinal column is a complex system of bones in stacked relation held upright by fibrous bands called ligaments and contractile elements called muscles. This column is critical for protecting the delicate spinal cord and nerves and for providing structural support for the entire body. There are seven bones in the neck (cervical) region, twelve bones in the chest (thoracic) region, and five bones in the low back (lumbar) region. There are also five bones in the pelvic (sacral) region which are normally fused together and form the back part of the pelvis. Each vertebra has a roughly cylindrical body with wing-like projections and a bony arch. The arches, which are positioned next to one another, create a tunnel-like space which houses the spinal cord. The anterior cylindrical bodies of the vertebrae, which are spaced apart by intervertebral discs, bear most of the compressive load of the spinal column. The spinal column is also flexible and is capable of a high degree of curvature and twist through a wide range of motion.
It is often necessary to surgically treat spinal disorders, such as scoliosis, as well as to surgically correct spinal problems such as those that occur due to trauma, developmental irregularities, or disease. Numerous systems are known for use in spinal correction and fixation, depending on the type of problem sought to be solved.
Spinal fusion (arthrodesis) devices attempt to restore stability to the spine by fusion in the problem area. These systems generally employ spinal instrumentation having connective structures such as one or more plates or rods that are placed on portions of the spinal column near the area intended to be fused. These systems usually include attachment devices including, but not limited to, pedicle screws, transverse process hooks, sublaminar hooks, pedicle hooks, and other similar devices. Rod systems, of which there are several, are frequently used in spine stabilization. Typically, the rods are utilized in pairs longitudinally along the length of the spinal column. For the sake of simplicity, the term “rod” will be used throughout to refer to any elongate or longitudinal member.
It is known that the strength and stability of a dual rod assembly can be increased by coupling the two rods with a cross-brace or connector that extends substantially perpendicular to the longitudinal axes of the rods across the spine. The simplest situation in which a connector could be used occurs when the two rods are geometrically aligned. Specifically, the two rods are parallel to each other, that is, there is no rod convergence or divergence in the medial-lateral direction. Stated alternatively, the two rods have the same orientation with respect to the coronal plane (viewed in the anterior-posterior direction); or, the rods are coplanar from a lateral view; and the two rods are located a uniform distance from each other.
In reality, spinal rods are rarely geometrically aligned in the above-mentioned simplest situation. The actual variations of geometrical alignment must be accommodated in some fashion. One way to accommodate actual arrangement is for one or both of the rods to be bent to accommodate the connector. However, any bending in either of the rods can adversely affect the fixation to the spine and compromise clinical outcome. Furthermore, the bending can adversely affect the mechanical properties of the rods, not to mention the fact that bending is both difficult and time-consuming for the surgeon. The connector can also be bent so that the disturbance to the rod positioning is minimized. Unfortunately, this too can cause the mechanical properties of the connector to be compromised.
To remedy these concerns, connectors with some adjustability have been designed to adapt for variations from the simplest geometrical alignment. One major problem with current devices is that those that do provide some form of length adjustability utilize inferior locking designs. Some utilize a slideable member with a pin anchor. Others use a slideable member with a compression style lock. The former style is cumbersome and runs the risk of pin-removal. The latter style is cumbersome and provides inadequate locking strength. In fact, most require the surgeon to impart a large amount of force on the construct in order to engage the lock. Despite engagement of these locking devices, none of these types of locking devices has been shown to adequately positively lock the length.
Another major problem with the current devices is that the method of locking the rod to the connector is inefficient or inadequate. Many current devices utilize threaded set screws that engage an exterior surface of the rod. Threading the set screw into the set screw opening applies a compressive force on the rod, which is supposed to secure the rod. Several problems exist with these threaded connections, including cross-threading, loosening over time, and the structural deformities imposed on the surface of the rod that is contacted by the set screw. Another current device uses a clamp body having opposable arms and utilizes a cam lug to force the arms closed in a scissors-like action to compressively load the rod. Yet another device utilizes a yoke-like clamping body disposed in a through-bore having resilient sidewalls that provide a wedging effect on the rod upon tightening of a locking screw in the through-bore. None of these devices, however, provide the simple, secure locking fit desired to positively retain a rod in situ long term.
An additional problem with these types of devices is that they are typically multi-piece systems that can be difficult to assemble and use in the surgical environment. And, even those that are one-piece designs do not allow for adjustments to compensate for all three modes in which there may be variation from geometrical alignment: convergence or divergence in the medial-lateral plane, non-coplanar rods, and variability in rod separation distances. For example, U.S. Pat. No. 5,947,966 discloses a device for linking adjacent spinal rods. In one embodiment, the device includes two members that are movable with respect to one another to accommodate different rod separation distances. A pin on one member engages a groove on the other member to provisionally couple the two members, thereby preventing a surgeon from separating the two members. Because the pin is sized to exactly fit the groove, no movement of the pin transverse to the longitudinal axis of the groove is possible. As a result, the device disclosed in the '966 patent cannot accommodate non-coplanar rods or adjust for rod convergence or divergence.
In some spinal surgeries, different types of devices are used to maintain the normal spacing between vertebrae, as well as to alleviate compression of the spinal cord. These devices are known as corpectomy devices and are typically inserted into a cavity created when all or a portion of one or more vertebrae are removed. One example of corpectomy devices are hollow mesh cages filled with bone chips or marrow, or even artificial bone material. Limitations of most present-day intervertebral implants are significant and revolve largely around the marked variation in disc space shape and height that results from either biologic variability or pathologic change. For example, if a disc space is 20 mm in height, a cylindrical implant bridging this gap requires a minimum height of 20 mm just to contact the end plate of the vertebral bone. Generally, end plate disruption must occur to allow a generous bony union, meaning that an additional 2-3 mm must be added on each end, resulting in a final implant size of 24-26 mm. During implantation from an anterior approach, excessive retraction is often required on the great blood vessels which significantly enhances the risk of devastating complications such as vascular tears or thrombosis. On the other hand, during a posterior approach, large implants may require excessive traction on neural elements for adequate placement, even if all posterior bony elements are removed. In some instances, an adequate implant size cannot be inserted posteriorly, particularly if there is a significant degree of ligamentous laxity requiring higher degrees of distraction to obtain stability by tightening the annular ligamentous tension band. Compromising on implant size risks sub-optimal stability or a loose implant, which has a greater chance for migration within or expulsion from the disc space. The alternative of excessively retracting neural elements to facilitate a posterior implant application results in a neuropraxia at best and permanent neural damage at worst.
Thus the need exists for an adjustable corpectomy that is simple to use in clinical procedures and that adequately and effectively spans the distance between vertebral bodies, is easily adjustable to account for space variability, and provides a secure lock once the desired dimension is achieved. Additionally, the need exists for an improved connector for spinal rods that can allow adjustability in all geometrical arrangements; that can provide quick and secure locking of the rod; and that provides a simple, positive locking length-adjusting mechanism that does not rely on compression fit or pin locking mechanisms.